The present invention generally relates to video cameras having an automatic focusing system, and more particularly to a video camera which has an automatic focusing system and is further provided with an optical type viewfinder.
Generally, there is a video camera having an automatic focusing system for automatically focusing on an object which is to be picked up. As is well known, there are various types of automatic focusing systems, such as the infrared type, the ultrasonic type, and the optical type which uses a part of the light from the object as the focusing information. These various types of automatic focusing systems have the known advantages and disadvantages, but recently, the optical type automatic focusing system is employed more frequently. In the present specification, the term "automatic focusing system" refers to the optical type automatic focusing system.
The video camera having the automatic focusing system, is designed so that a part of the light from the object is separated from a main optical system which directs the light from the object to a camera tube, and this separated light is supplied to the automatic focusing system as the focusing information. Accordingly, the quantity of light which reaches the camera tube, is reduced by the quantity of light which is separated from the main optical system for use as the focusing information. In order to be able to satisfactorily pick up the image of the object even under a dark condition, the quantity of light reaching the camera tube should be large, but on the other hand, the quantity of light separated from the main optical system should be large to a certain extent so as to obtain a positive focusing information for use in the automatic focusing system. Hence, 70% of the incoming light to the main optical system from the object is conventionally supplied to the camera tube, and the remaining 30% of the incoming light is supplied to the automatic focusing system.
The video camera is provided with a viewfinder, so that the operator can view the object through the viewfinder and determine the framework of the picture.
As one type of the viewfinder, there is the electronic or electric type viewfinder. This electronic viewfinder has a compact cathode-ray tube (CRT), and a video signal related to the image which is picked up by the camera tube is also supplied to the CRT so that the reproduced picture can be monitored. Since this viewfinder is not the optical type and does not use light, the quantity of light reaching the camera tube and the automatic focusing system will not be reduced when this viewfinder is applied to the video camera having the automatic focusing system. However, this type of viewfinder uses the CRT, and for this reason, the cost of the viewfinder is high and the power consumption of the viewfinder is large. Further, a black-and-white CRT is generally employed for the viewfinder, and it is impossible to check the state of the color. When a color CRT is employed for the viewfinder, the cost of the viewfinder becomes even higher. In addition, because the picture on the compact CRT is viewed through a magnifying lens, the scanning lines are conspicuous and the picture is unclear.
On the other hand, there is a video camera which is provided with another optical system in addition to the main optical system for directing the light from the object to the camera tube. The image of the object is obtained in the viewfinder by passing the light from the object through this other optical system. The disadvantages described before are eliminated according to this video camera, however, a gap is formed between an objective lens of this other optical system which obtains the image in the viewfinder and an objective lens of the main optical system which directs the light to the camera tube. As a result, a parallax occurs due to the gap.
There is also a video camera which employs the so-called through-the-lens (TTL) type viewfinder. In the TTL type viewfinder, a part of the light from the object is separated from the main optical system which directs the light from the object to the camera tube, and this separated light is supplied to the viewfinder system. However, this TTL type viewfinder uses a part of the light reaching the camera tube. Hence, when this TTL type viewfinder is applied to the video camera having the automatic focusing system of the type described before, the quantities of light reaching the TTL type viewfinder, the camera tube, and the automatic focusing system become greatly reduced. Therefore, conventionally, there was no concept of providing the TTL type viewfinder and the automatic focusing system in the video camera.